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Common Exam 1 Feb. 27th, Wednesday Time: To Be Announced Room: To Be Announced Check your NJIT e-mail and course website HW due every Monday Quiz every Thursday Today Newton’s Second Law of Motion Newton’s Third Law of Motion Forces Gravity force (weight) Normal force 1 Chapter 4 The Laws of Motion Last class…. Newton’s First Law of Motion If the net force on an object is zero, An object at rest remains at rest An object moving with constant velocity continues to move with the same speed and in the same direction (the same velocity). In equation, G If Fnet = 0, G v does not change. An additional force is NOT required to keep a cart moving with a constant speed on a frictionless track! 2 What if the net force on an abject is NOT zero? Æ Newton’s Second Law of Motion Newton’s Second Law of Motion • Unbalanced forces cause object to accelerate. • An object of mass m has an acceleration a, equal to the net force F, divided by the mass of the object, m. G F ∑ G F G a= = net m m 3 G ∑F G G Fnet G = ⇒ Fnet = ma m m m [1N ] = [1kg ] × 1 2 s G a= kg m s2 SI unit of force is a Newton: 1N ≡ 1 Non-SI unit for force: pound (lb) 1 N = 0.225 lb Newton’s 2nd law for each component Fnet , x = ma x ; Fnet , y = ma y ; Fnet , z = ma z Quick quiz True or false? If the net force acting on an object is in the positive x-direction, the object moves only in the positive x-direction. Answer: False For example, consider projectile motion. Example 1: Force and Acceleration Æ see note 4 Equal magnitude and opposite direction Newton’s Third Law of Motion • Forces always appear in pairs: for every force exerted by object A on object B there is an equal and opposite force exerted by B on A (action-reaction pair). • The 3rd law pair of forces are always applied to different objects! (Net force in 1st and 2nd laws is the sum of forces on the same object.) G G FA on B = − FB on A 5 Find reaction force for the gravity on the cantaloupe Since the gravity is Force on the cantaloupe from the earth…. Answer: (Gravity) Force on the earth from the cantaloupe ! 6 NOT the action-reaction pair! Forces: Gravitational Force: ¾ F = mg Archimedes Force ¾ Friction Force: ¾ Tension Force ¾ Spring Force ¾ ¾ Normal Force 7 Gravitational Force (Weight): Weight: From the Earth on objects near Earth Surface Magnitude: F = mg, where g = 9.8 m/s2 Direction: Pointing downward •Weight: The force that the Earth is pulling the object with near the surface of the Earth. •Weight is a vector quantity, it has a magnitude and direction (unit : N) (weight) = (mass) x g Direction: Magnitude: Determined by analyzing forces From surface on objects in contact 8 Example 2: Person in an elevator Picture of Situation A person with a mass 50 Kg in an elevator is moving upward 2 with an acceleration a = 1.1 m / s Identify all forces on the person and find magnitude and direction for each. Example 3: Object on an inclined surface (see note) 9 Today, we learned Newton’s Second Law of Motion Newton’s Third Law of Motion Forces Gravity force (weight) Normal force 10